Taste for salt tied to drug addiction

Scientists suggest same systems in brain responsible for cravings for both.

By Don Finleydfinley@express-news.net

Updated 11:35 pm, Sunday, July 24, 2011

When the first slimy amphibians crawled onto dry land a few million years ago, they brought with them a powerful craving from that salty, primordial sea.

How powerful? It turns out that the genes and neural networks in the brain that regulate hunger for salt seem to be the same ones at play in drug addiction. And that could have some far-reaching implications — helping to explain why narcotics addiction is so hard to treat and maybe why people are so drawn to some pretty unhealthy foods, said an international team of scientists with local ties.

“The desire to ingest salt is an instinct that has been known for a while,” said Dr. Wolfgang Liedtke, an assistant professor of neurobiology at Duke University and one of the lead authors of a new study. “But now comes this notion that salt appetite uses pathways that also have been taken advantage of by cocaine and opiate addiction. That helps us understand why the lust to gratify salt appetite has such a powerful influence on human behavior.”

Salt is a critical part of diet, maintaining healthy fluid levels in the body. And it's important for muscle and nerve function. Beyond that, its ability to keep food from rotting helped forge early civilizations. Roman soldiers were paid with it.

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Yet inland, the mineral is relatively hard to come by. Early humans learned to mine it from the earth. Meat-eating animals get salt from the flesh of their prey. But veggie-loving animals sometimes go to great lengths to get it.

Liedtke points to mountain goats in Italy that scale an almost vertical dam to reach salt deposits in the rock, and a herd of elephants in Kenya that learned over generations to march single-file a mile into a dark cave to reach a salt lick.

“And all of that, just to lick salt,” he said. “It's crazy, but that's what they do.”

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And while the researchers aren't suggesting that a craving for salt makes one more likely to crave narcotics, Liedtke said that ancient, ingrained desire for the salty might explain why drug addiction is so stubborn to overcome.

“Our findings imply that abstinence-aimed therapies are up against reward systems that have evolved over hundreds of millions of years, thus conferring a powerful survival advantage,” Liedtke said.

The discovery, published this month in the Proceedings of the National Academy of Science, that the mechanisms in the brain were similar in salt appetite and drug addiction came from studying how genes turned on and off when rats were fed or deprived of salty water.

In some cases, medication was used to increase the animals' hunger for salt. In others, they were given a drug to block the effects of dopamine — a chemical messenger in the brain also linked to drug addiction — which decreased the animals' desire for salt.

That activity was in the hypothalamus, a part of the brain that controls hunger, thirst and sleep. In particular, the scientists found a link to orexin, a substance that's been implicated in appetite, sleep and addiction.

Also taking part in the study was Dr. Donald Hilton, clinical associate professor of neurosurgery at the University of Texas Health Science Center, who has a “side interest” in addiction and recently co-wrote a controversial paper on the parts of the brain that might be responsible for pornography addiction.

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Some addiction experts long have speculated that narcotics addiction might have piggybacked onto some ancient system of craving. They include Dr. George Koob, who studies the neurobiology of addiction disorders at Scripps Research Institute in La Jolla, Calif.

“I think it's really exciting,” said Koob, who didn't take part in the study. “They're studying a basic, metabolic drive that converts into motivated behavior. And they link it to the hypothalamus and orexin.”

Laura Almasy, a geneticist at Texas Biomed who studies the genetics of addiction and also wasn't involved in the study, agreed that the findings make sense.

“Addiction is damaging, but the way it starts is that someone takes a substance and it feels good, so they do it again,” Almasy said. “And I think what this paper suggests is that the mechanism for why it feels good is that cocaine and opioids are hitting the pathways that were laid down to help us regulate salt intake.”